Bone fixation device

ABSTRACT

A bone fixation device includes a plastically deformable bone fixation body having a screw receiving aperture extending therethrough. The shape of the aperture is defined by a side wall having a first threaded portion, a second smooth walled portion extending from the first threaded portion, and a third smooth walled portion extending from the second portion. The aperture has a step change in cross section between the second and third portions defining a lip. The second and third portions are connected by a lip face extending substantially normal to the aperture length with the edge of the lip face defining the lip. The device further includes at least one bone fixation screw having a shaft and a head. The shaft engages with the first threaded portion of the aperture and the head abuts the lip with an abutment force being directed substantially radially outwards so producing a tight fit.

The subject patent application is a continuation-in-part of co-pendingU.S. patent application Ser. No. 13/098,087, filed on Apr. 29, 2011,which is a continuation-in-part of U.S. patent application Ser. No.12/302,586, filed on May 26, 2009, now abandoned, which is the NationalStage and claims priority to and all the advantages of InternationalPatent Application No. PCT/GB07/01912, filed on May 21, 2007, whichclaims priority to U.K. Application No. 0610630.6, filed on May 26,2006, the disclosures of which are hereby incorporated by reference.

The present invention relates to a bone fixation device. Moreparticularly, but not exclusively, the present invention relates to abone fixation device having at least one aperture extendingtherethrough, and a bone fixation screw. The screw and the aperture eachcomprise a threaded portion and a smooth portion, the smooth portion ofthe screw being adapted to radially abut the smooth portion of theaperture as the respective threaded portions are threaded intointerengagement.

Bone fixation devices having apertures extending therethrough forreceiving bone fixation screws are known. U.S. Pat. No. 5,336,224discloses a bone fixation device comprising a plurality of screwreceiving members connected together in a line by connecting arms.Extending through each screw receiving member is a screw receivingaperture. Each aperture comprises a threaded portion and a bowl shapedsmooth portion. In use a screw is threaded into engagement with thescrew receiving aperture. As the screw head is received in the bowl, thehead urges the bowl forwards in the direction of the axis of the screwuntil the bone fixation device abuts the bone. The head is thentightened in the bowl.

Such a bone fixation device must be perfectly matched to the profile ofthe bone. If the two are not perfectly matched then as one screw istightened urging the bone fixation device into contact with the bonethen displacement of the bone fixation device may pull a previouslytightened screw out of engagement with the bone. This can be aparticular problem for elderly patients or patients with low bonedensity.

The bone fixation device according to the invention seeks to overcomethe problems of the prior art.

Accordingly, in a first aspect, the present invention provides a bonefixation device comprising

-   a plastically deformable bone fixation body having at least one    screw receiving aperture extending therethrough;-   the shape of the aperture being defined by a side wall, the side    wall comprising a first threaded portion, a second smooth walled    portion extending from the first threaded portion and a third smooth    walled portion extending from the second smooth walled portion;-   the aperture having a step change in cross section between the    second and third smooth walled portions defining a lip for abutment    with the head of the screw, the step change being a discontinuity in    the size of the aperture at the lip with the third portion being    larger than the second portion, the second and third portions being    connected by a lip face which extends substantially normal to the    length of the aperture, the edge of the lip face defining the lip;    and,-   at least one bone fixation screw, the screw comprising a threaded    shaft and a head, the shaft being adapted to engage with the first    threaded portion of the aperture and the head being adapted to abut    the lip with an abutment force, the abutment force being directed    substantially radially outwards so producing a tight fit.

As a bone fixation screw is threaded into engagement with the screwreceiving aperture of the bone fixation device according to theinvention the head of the screw abuts the lip. The combination ofthreaded engagement of screw receiving aperture and screw and abutmentof screw receiving aperture and screw head firmly locks the twotogether. The bone fixation device may therefore be arranged remote fromthe bone and locked in place by bone fixation screws. The bone fixationdevice therefore does not need to be exactly the same profile as thebone, considerably simplifying the surgical procedure.

Each of the portions of the aperture can be cylindrical.

The diameter of the first threaded portion can be less than the diameterof the second smooth portion.

There can be a step change between the first threaded portion and secondsmooth portion.

The second smooth portion can comprise a conical portion extending fromthe first threaded portion to a region of constant diameter of thesecond smooth portion.

The bone fixation body can comprise at least two screw receiving memberseach having a screw receiving aperture extending therethrough, the screwreceiving members being connected together by a deformable connectingarm.

The bone fixation body can comprise a plurality of screw receivingmembers connected together in a line by a plurality of deformableconnecting arms.

The screw receiving members can be substantially spherical.

A portion of the substantially spherical member can be flattened, theflattened portion being centered about a mouth of the aperture and beingin a plane normal to the axis of the aperture.

The bone fixation body can be a plate.

The bone fixation body can comprise first and second laminar portionsconnected together by a step.

The side walls of the aperture can extend from the body.

In a further aspect of the invention there is provided a bone fixationdevice comprising

-   a plastically deformable bone fixation body having at least one    screw receiving aperture extending therethrough, wherein the    aperture is cylindrical and of constant area along its length;-   the shape of the aperture being defined by a side wall, the side    wall comprising a first threaded portion and a second smooth walled    portion extending from the first threaded portion; and,-   a bone fixation screw having a threaded shaft and a head, the shaft    being adapted to engage with the first threaded portion to draw the    head into the aperture, wherein the screw head is tapered with the    diameter of the head increasing in a direction away from the    threaded shaft;-   the taper of the head of the screw being adapted such that as the    screw head is drawn into the aperture the head abuts the second    smooth walled portion with an abutment force which is directed    substantially radially outwards, so locking the screw and aperture    together.

Again, as the bone fixation screw is threaded into the screw receivingaperture the screw head abuts the side wall of the aperture locking thebone screw and bone fixation device together.

The plastically deformable bone fixation body can comprise a pluralityof screw receiving members connected together in a line by connectingarms.

The screw receiving members can be substantially spherical.

A portion of the substantially spherical member can be flattened, theflattened portion being centered about a mouth of the aperture and beingin a plane normal to the axis of the aperture.

The bone fixation body can be a plate.

The bone fixation body can comprise first and second laminar portionsconnected together by a step.

The present invention will now be described by way of example only andnot in any limitative sense with reference to the accompanying drawingsin which

FIG. 1 shows a known fixation plate before and after deformation;

FIG. 2 shows a bone fixation device according to the invention inperspective view;

FIGS. 2A and 2B show the bone fixation device of FIG. 2 in combinationwith a screw;

FIG. 3 shows the embodiment of FIG. 2 in combination with a screw;

FIG. 4 shows a further embodiment of a bone fixation device according tothe invention in cross sectional view;

FIG. 5 shows the embodiment of FIG. 4 after twisting;

FIG. 6 shows a further embodiment of a bone fixation device according tothe invention in perspective view;

FIG. 7 shows a cross section through a screw receiving aperture of thedevice of FIG. 6;

FIG. 8 shows a further embodiment of a bone fixation device according tothe invention; and,

FIG. 9 shows a further embodiment of a bone fixation device according tothe invention.

Shown in FIG. 1 is a known bone plate 1. The bone plate 1 comprises ametal plate 2 having a plurality of apertures 3. In use the bone plate 1is placed against a bone (not shown). Bone fixation screws (not shown)are passed through the apertures 3 and then screwed into the bone. Thescrew heads typically engage with the plate 1 firmly fixing the plate 1in position and preventing it from being displaced with respect to thebone.

A problem can arise however if the bone plate 1 needs to be deformedbefore it can be fixed to the bone. Deformation of the plate 1 deformsthe apertures 3 in the plate 1 as shown. This prevents the screw headsfrom accurately engaging with the apertures 3 which may result in theplate 1 being free to wobble slightly with respect to the bone. This canreduce the effectiveness of the bone plate 1 as a support for the bone.

In addition, it can be difficult to remove such a known bone plate 1from the bone when it is no longer required. The screw heads tend tocold weld to the bone plate 1 over time making the screws difficult toremove. It is often necessary to cut the bone plate 1 free which canresult in damage to the bone.

Such a known bone plate 1 is also limited as to how it can be deformed.Whilst the plate 1 can be bent as shown in FIG. 1 it is not a simplematter to twist it such that the apertures 3 lie in different planes.

In addition, such a known bone fixation plate 1 comprises relativelysimple threaded apertures. The fixation plate is screwed in place bythreading screws into engagement with the apertures. The heads of thescrews urge the bone fixation plate into abutment with the bone.

In use a surgeon must deform the plate to exactly the same profile asthe bone so that all of the screws can be fully tightened. This can betime consuming and difficult for the surgeon. If the curvature of thebone fixation plate does not match the bone exactly then as one screw istightened the plate may move causing a screw further along the plate tobe pulled out of engagement with the bone.

Shown in FIG. 2 is a bone fixation device 8 according to the invention.The device comprises a plastically deformable bone fixation body whichcomprises a plurality of screw receiving members 9. Apertures 10 extendthrough each of the screw receiving members 9 for receiving screws. Along axis 11 extends between each of the screw receiving members 9.Plastically deformable connecting arms 12 extend along the long axis 11between the screw receiving members 9 as shown. In this embodiment thelong axis 11 passes through the apertures 10 of the screw receivingmembers 9.

In use the bone fixation device 8 is gripped and bent to the requiredshape. The arms 12 are more pliable than the screw receiving members 9and accordingly it is the arms 12 that bend when the force is applied,rather than the screw receiving members 9. The apertures 10 thereforeremain undistorted. In addition, in contrast to known bone plates 1 atorsional (twisting) force can be applied to the device 8 rotating oneor more of the screw receiving members 9 about the long axis 11 of thedevice 8 if required. As the long axis 11 passes along the length of theconnecting arm 12, the connecting arm 12 twists about its length. Thedevice 8 can therefore be twisted without significantly altering itsdimensions. The device 8 can therefore be inserted into small apertureseven after twisting.

In this embodiment of the invention each of the screw receiving members9 is substantially spherical with the apertures 10 extending through thecenters of the spheres 9. Each aperture 10 intersects the sphere atmouths 13 on opposite sides of the sphere 9. The sphere 9 comprises aslightly flattened portion 14 around one of the mouths. This reduces theprofile of the device 8. It also provides an extended contact areabetween the screw receiving members 9 and the bone.

The connecting arms 12 between the screw receiving members 9 arecylindrical. The interface 15 between the arms 12 and spherical screwreceiving members 9 is chamfered so that any bending or torsional forcesdo not concentrate at this interface 15.

The ends 16 of the device 8 are tapered as shown so that the device 8can be placed between bone and soft tissue without surgically exposingthe entire length of bone.

As shown in FIGS. 2A and 2B, each of the apertures 10 of this embodimentis cylindrical having a constant area along its length. A portion of theaperture 10 is threaded. The remainder of the aperture is smooth walled.

After bending and/or twisting to the correct shape the device 8 ispositioned against the bone. Screws are inserted into the apparatus 10through the smooth portions and into threaded engagement with thethreaded portions. On further rotation of the screws they penetrate andgrip the bone, fixing the device 8 to the bone. A significant advantageof the device 8 is that is can be bent/twisted to the correct shape,positioned correctly and then the screws inserted. This considerablysimplifies the attachment procedure. As the screw receiving members 9are aligned with the connection arms 12 along the long axis 11 thedevice 8 can be twisted about its length without any significant changein dimensions of the device 8. This is particularly useful wheninserting the device 8 into small apertures.

The device 8 is adapted to be used with a screw (not shown) having twoportions—a threaded portion for gripping the threaded portion of theaperture 10 and then the bone and a smooth head portion extending fromthe threaded portion. The smooth head portion has an outer face which issubstantially cylindrical and of the same diameter as the threadedportion. The smooth head portion is however slightly tapered with itsdiameter increasing in a direction away from the threaded portion. Atits end the diameter of the smooth head portion is slightly larger thanthe diameter of the aperture 10. As the screw is turned and is drawninto the aperture 10 the smooth head portion of the screw abuts thesmooth portion of the aperture 10 so producing a press fit. This pressfit firmly locks the bone fixation device 8 and screw together, evenwhen the device 8 is remote from the bone. The bone fixation device 8therefore does not need to exactly match the profile of the bone. Thisconsiderably simplifies the surgical procedure. The taper of the smoothhead portion is very slight. Accordingly, as the head portion abuts (ieis pushed into contact with) the smooth portion this produces anabutment force which extends substantially radially outwards, so lockingthe screw and aperture together.

An alternative embodiment of the invention is shown in FIG. 3. In thisembodiment the threaded portion of the aperture 10 is narrower indiameter than the smooth portion. The screw 17 has a narrow threadedportion 18 and a wider smooth tapered head portion 19 as shown. Thenarrow threaded portion 18 of the screw 17 engages with the narrowportion of the aperture 10 drawing the larger smooth head portion 19into abutment with the smooth portion of the aperture 10.

In the above embodiments the smooth portions are perfectly smooth. Inalternative embodiments the smooth portions may comprise surface texturesuch as ripples or bevels. In a further embodiment of the invention aportion of the aperture 10 is slightly bevelled so that a standard bonescrew head will press fit into engagement with the bevels as the screwis drawn into the aperture 10.

Shown in FIG. 4 in cross sectional view is a further embodiment of adevice 8 according to the invention. The aperture 10 comprises a firstnarrow threaded section 20. Extending from this is a second smoothwalled section 21 of larger diameter. Extending from the second section21 is a third smooth walled section 22 of slightly larger diameter thanthe second section 21. The step change in diameter from the secondsection 21 to the third section 22 defines a lip 23. In use a screw 24is inserted into the aperture 10 with the threaded portion 25 of thescrew 24 in threaded engagement with the narrower portion 20 of theaperture 10. As the screw 24 is turned and drawn in to the aperture 10the head 26 of the screw 24 abuts the lip 23 producing a tight fit.

Shown in FIG. 5 is the embodiment of FIG. 4 in perspective view. Thedevice 8 has been twisted along its length such that the apertures 10lie in different planes. As can be seen, the arms 12 have twisted beforethe screw receiving members 9 deform.

In a further embodiment of the invention the screw receiving members 9are substantially elliptical.

The device 8 according to the invention can be used with any tool whichgrips the screw receiving members 9. One preferred embodiment of such atool comprises jaws having cut out sections which match the sphericalcomponents of the screw receiving members 9. The tool also has cut outswhich match the interface 15 between the spherical component 9 andconnecting arm 12 so that the tool can apply a bending force throughoutthe length of the bone fixation device 8. In an alternative embodimentthe jaws have cut outs which match the flattened portions 14 of thescrew receiving members 9.

In use two of the tools are used to grip the screw receiving members 9.The device 8 is then bent and/or twisted to the desired shape and thenreleased.

Shown in FIG. 6 is a further embodiment of a bone fixation device 8according to the invention in perspective view. In this embodiment theplastically deformable bone fixation body comprises a thick rectangularplate 30. Arranged along the plate 30 and extending through it is aplurality of screw receiving apertures 10 as shown. In use the plate 30is deformed to approximately the same profile as the bone and thenscrewed to the bone by screws inserted through the apertures 10. Thebone screws are tightened until the screw head abuts the wall of theaperture 10 as described in further detail with reference to FIG. 7.

Shown in FIG. 7 is a cross section through one of the screw receivingapertures 10 of the bone fixation device of FIG. 6. The aperture 10comprises a first cylindrical threaded portion 31. Extending from thefirst portion 31 is a smooth cylindrical second section 32. The secondsection 32 comprises a conical portion 33 and then a uniform diameterportion 34. Extending from the second section 32 is a third smoothwalled section 35. There is a step in cross section between the secondand third sections 32, 35 defining a lip 36.

In use a screw comprising a threaded shaft and a head is inserted intothe aperture 10. The threaded shaft is threaded into engagement with thefirst threaded portion 31 of the aperture 10. As the screw is turned thehead is drawn into the aperture 10. The threaded shaft extends throughthe first threaded portion 31 and into the bone. As turning continuesthe head radially abuts the lip 36 locking the screw and bone fixationdevice 8 together. The head of the screw may be a domed head.Alternatively, the head of the screw may be frustoconical with thediameter of the head increasing in a direction away from the threadedshaft.

Shown in FIG. 8 is a further embodiment of a bone fixation deviceaccording to the invention. In this embodiment the plasticallydeformable bone fixation body comprises two laminar members 40,41connected together by a step 42. Each of the laminar members 40,41comprises a plurality of screw receiving apertures 10.

Shown in FIG. 9 is a further embodiment of a bone fixation device 8according to the invention. In this embodiment the bone fixation body isa laminar plate 50. The plate 50 is substantially triangular in planview having rounded corners. In this embodiment the side walls of thescrew receiving apertures are upstanding from the plate 50 as shown suchthat the depth of the apertures 10 is greater than the thickness of theplate 50.

Embodiments in which the bone fixation body is a combination of one ormore fixation bodies as described above. For example the body fixationbody can be a combination of a plate portion and a further portioncomprising a plurality of screw receiving members connected together byarms.

Returning to FIG. 7, as previously described the aperture comprisessecond and third smooth walled portions 32,35. Both of these portions32,35 are circular in cross section in a plane normal to the length ofthe aperture 10. At the join between the second and third smooth walledportions 32,35 there is a discontinuity (or in other words a stepchange) in the size of the aperture 10 with the third smooth walledportion 35 being larger than the second smooth walled portion 32. Theradius of the third smooth walled portion 35 is larger than the second32. The second and third smooth walled portions 32,35 are connectedtogether by a lip face 52. The plane of the lip face 52 is substantiallynormal to the length of the aperture 10. The angle between the side wallof the second smooth walled portion 32 and the lip face 52 issubstantially 90 degrees. The edge of the lip face 52 at its point ofcontact with the second smooth walled portion 32 defines a lip 36.

In use the threaded portion of the screw is threaded into engagementwith the threaded portion 31 of the aperture 10. As the screw headenters into the aperture 10 the screw head abuts (ie is pushed intocontact with) the lip 36. This produces an abutment force. The abutmentforce is directed substantially radially outwards so producing a tightfit.

The invention claimed is:
 1. A bone fixation device comprising aplastically deformable bone fixation body having at least one screwreceiving aperture extending therethrough, wherein the aperture iscylindrical and of constant area along its length; the shape of theaperture being defined by a side wall, the side wall comprising a firstthreaded portion and a second smooth walled portion extending from thefirst threaded portion; and, a bone fixation screw having a threadedshaft and a head, the shaft being adapted to engage with the firstthreaded portion to draw the head into the aperture, wherein the screwhead is tapered with the diameter of the head increasing in a directionaway from the threaded shaft; the taper of the head of the screw beingadapted such that as the screw head is drawn into the aperture the headabuts the second smooth walled portion with an abutment force which isdirected substantially radially outwards, so locking the screw andaperture together.
 2. A bone fixation device as claimed in claim 1,wherein the plastically deformable bone fixation body comprises aplurality of screw receiving members connected together in a line byconnecting arms.
 3. A bone fixation device as claimed in claim 2,wherein the screw receiving members are substantially spherical.
 4. Abone fixation device as claimed in claim 3, wherein a portion of thesubstantially spherical member is flattened, the flattened portion beingcentered about a mouth of the aperture and being in a plane normal tothe axis of the aperture.
 5. A bone fixation device as claimed in claim1 wherein the bone fixation body is a plate.
 6. A bone fixation deviceas claimed in claim 1, wherein the bone fixation body comprises firstand second laminar portions connected together by a step.